1. Field of the Invention
The present invention relates to a manual transmission for a four wheel drive vehicle and, in particular, to a manual transmission having a small agitation resistance of lubrication oil and having a differential mechanism which can vary a torque distribution ratio between input output elements according to running conditions.
2. Description of Prior Arts
Generally, the manual transmission used for a four wheel drive vehicle is constituted by a manual transmission gearing mechanism for changing speeds and a driving force distributing mechanism for distributing driving force into front and rear wheels. For example, Published Japanese Patent Application No. Toku-Kou-Hei 4-55892 discloses a transmission technique as shown in FIG. 25, in which numeral 201 denotes an input shaft rotatably mounted on a transmission case 202 for transmitting driving force of an engine to the transmission through a clutch. Numeral 203 denotes a front drive shaft arranged in parallel with the input shaft 201 and rotatably supported by the transmission case 202.
The front drive shaft 203 is rotatably fitted to a hollow-shaped shaft 205 which is driven by the input shaft 201 through a manual transmission gearing mechanism 204. The hollow-shaped shaft 205 is divided into an input side shaft 205a and an output side shaft 205b through a sleeve 206 and it is connected at the output end thereof with a driving force distributing apparatus 207.
The driving force distributing apparatus 207 comprises a pinion shaft 208 fixed to the output end of the hollow shaft 205, a pinion 209 rotatably supported by the pinion shaft 208, and side gears 210a and 210b meshing with the pinion 209. On the other hand, the side gear 210a is spline-fitted to a collar 211 which is connected with a rear end of the front drive shaft 203.
On the other hand, the side gear 210b is integrally formed with a differential case 212 for accommodating the pinion shaft 208, the pinion 209 and side gears 210a, 210b. The differential case 212 is rotatably supported at both ends thereof by the an extension case (not shown).
Further, the differential case 212 has a drive gear 213 through which driving force is transmitted to a rear drive shaft (not shown).
Further, the driving force inputted from the engine to the input side shaft 205a of the hollow shaft 205 through the input shaft 201 and the manual transmission gearing mechanism 204 is transmitted to the pinion shaft 208 through the sleeve 206 and the output side shaft 205b. The driving force is divided into two, one is transmitted to the front drive shaft 203 through the pinion 209, the side gear 210a and the other is transmitted to the rear drive shaft (not shown) through the pinion 209, the differential case 212 and the drive gear 213. Thus, the driving force transmitted to the front drive shaft 203 drives front wheels and the one transmitted to the rear drive shaft drive rear wheels. When a rotational difference is generated between the front and rear drive shafts, a rotational difference occurs between the side gear 210a on the front drive shaft 203 side and the side gear 210b on the rear drive shaft side and as a result the rotational difference is absorbed by the rotation of the pinion 209.
Since the transmission according to this prior art is constituted such that the driving force of the engine is transmitted directly to the rear drive shaft side through the pinion shaft, the pinion and the side gear, it is not necessary to provide an additional intermediate shaft for transmitting driving force from the side gear to the rear drive shaft and resultantly the compact transfer case can be realized. Further, since the rear drive gear is provided coaxially with the front drive shaft through the hollow shaft, the rear drive shaft can be arranged above the front drive shaft, therefore it becomes easier to obtain a large road clearance in consideration of facilitating an off-road running.
Further, the transmission according to this prior art has an advantage of being able to lubricate the front differential, the manual transmission gearing mechanism, the driving force distributing apparatus with a single lubrication oil because these main components are arranged almost at the same level. However, since it is generally needed that the static oil level comes at least near the rotating axis of the hypoid gear of the front differential, most parts of the driven gears of the manual transmission gearing mechanism and the driving force distributing apparatus dip in the lubrication oil.
When the vehicle is operated in this state, the hypoid gear having a relatively large diameter and the driving force distributing apparatus disposed behind the hypoid gear pump up the lubrication oil. This "pumping-up" phenomenon becomes greater with an increase of the vehicle speed. In high speeds, most lubrication oil is attracted to the hypoid gear and the driving force distributing apparatus. As a result of this, the oil level around the manual transmission gearing mechanism becomes low, leading to insufficient lubrication. Generally, to solve this problem, the static level of lubrication oil must be raised to a higher level.
Thus, the "pumping-up" phenomenon brings about a great amount of lubrication oil and as a result power transmission efficiency and fuel economy are exacerbated due to an increase of agitation resistance of oil. The magnitude of this exacerbation becomes large as the vehicle speed becomes high. Further, when the vehicle is accelerated or runs on a grade, the lubrication oil moves towards the driving force distributing apparatus and causes an increase of agitation resistance of oil, this also leading to an adverse effect on fuel economy.
Further, this increase of stirring resistance brings about an increase of oil temperature leading to deterioration of oil. The deterioration of oil incurs wearing or damages of the manual transmission mechanism in gears, damages of the synchromesh mechanism and the driving force distributing apparatus or damages of oil seals in lip surfaces.
Further, since the driving force distributing apparatus is mounted at the rear end of the front drive shaft, that is, at the rear end of the transmission, the driving force distributing apparatus is formed being projected and further this projection restricts a freedom in installing members for supporting the transmission or establishing spaces for accommodating a transmission control system or an exhaust system. This results in making interchangeability between a manual transmission and an automatic transmission more difficult.